During the manufacture of semiconductors, photoresist liquids must be precisely and accurately dispensed and deposited on the wafer being treated. In conventional apparatus for photoresist application, wafers to be processed are positioned beneath a suitable nozzle which then dispenses a predetermined amount of photoresist liquid to coat the wafer. Typically the wafer is then rotated to disperse the deposited liquid evenly over the entire surface of the wafer. It is readily apparent that the rate of dispensing and the amount of liquid dispensed are critical in this process.
When fluid flow is stopped through the nozzle, such as between wafer treatments, the potential exists for droplets of photoresist liquid from the nozzle to form and fall onto the wafer positioned below the nozzle. This can destroy the pattern being formed on the wafer, requiring that the wafer be discarded or reprocessed.
In order to avoid the formation of deleterious droplets on the nozzle, external stop/suckback valves are commonly used. Such valves are typically a dual pneumatically controlled valve pair, with one valve stopping the flow of liquid to the nozzle, and the other drawing the liquid back from the dispense end or outlet port of the nozzle. This not only helps prevent droplet formation and dripping at the port, but also helps prevent drying of the exposed surface of the liquid, which can lead to clogging of the nozzle, and reduces fluid contamination at the outlet.
The rate at which each of these stop/suckback valves closes or opens effects the fluid in different ways. If the stop valve opens or closes too fast, the fluid column can xe2x80x9cspitxe2x80x9d a droplet of fluid onto the wafer, deleteriously altering the coating thickness. If the suckback valve is changed too quickly, the fluid column will cavitate, creating a bubble in the column, again deleteriously altering the thickness of the resulting coating on the wafer.
One approach used to address this problem is the use of needle valves in series with the pneumatic input and exhaust lines. The needle valves provide regulation of the pressure change at the valve. However, this method of controlling the valve has various pitfalls. For example, the cleanliness of the controlling gas is an issue, as is the likelihood of dirt collecting on the valve seat. Similarly, any variation of the incoming pressure can decrease the reliability of this approach, as can mechanical vibration. Over time, any of the foregoing drawbacks can alter the valve performance. As a result, the liquid dispense must be monitored and manually adjusted periodically. This can be time-consuming and is not cost effective.
The coating of larger wafers (e.g., 300 mm in diameter and larger) is also problematic, as turbulence issues arise. The rotational speed of the wafer is conventionally used to spread the coating fluid from the center of the wafer where it is applied, radially outwardly to the edge of the wafer. However, this approach creates turbulent air flow over the wafer and can result in uneven or nonuniform coatings. Reducing the spin speed with larger wafers reduces the turbulence at the surface of the wafer, but can instroduce new problems. With the reduced speed, the fluid moves slower across the wafer, and thus spreading the fluid to the wafer edge before the fluid begins to setup or dry becomes an issue.
It therefore would be desirable to provide a stop/suckback valve system that results in precise, reproducible dispensing of fluid without the foregoing disadvantages. In addition, the present invention has broader applications to any pneumatic fluid control device, especially where precise control of fluid flow is desired or required.
It would be further desirable to provide a control system that allows for multiple dispense points on a substrate such as a large wafer, in order to reduce or eliminate the turbulence issues that arise when processing larger wafers, and to provide uniform coatings on the wafers.
The problems of the prior art have been overcome by the present invention, which provides a control system for monitoring (preferably digitally) and/or controlling pressure to a pneumatic load such as a stop/suckback valve. Pressure is sensed in the pneumatic fluid input lines to the pneumatic load, and a closed loop control is used to maintain the pressure at a predetermined level. The pressure(s) can be continuously or continually monitored, and valves modulated to obtain the desired result. The control system has applicability to fluids having a wide range of viscosities, it being capable of accurately and repeatably dispensing such fluids with minimal operator involvement.
In an alternative embodiment, the control system is used to ensure equal dispensing in multi-point dispense apparatus. Thus, pressure is sensed downstream of each valve in fluid communication with a respective nozzle. Any pressure differential is indicative of unequal dispense rates through the nozzle, and is compensated for by appropriate valve adjustment.
In yet another embodiment of the present invention, multiple dispense points are used to dispense a coating fluid onto a substrate simultaneously or sequentially. This will decrease the time needed to coat a large substrate such as a wafer, since the dispense occurs at different points along the substrate surface that can be controlled by the user.